Sp3 bonding boron nitride nitride thin film having self-forming surface shape being advantageous in exhibiting property of emitting electric field electrons, method for preparation thereof and use thereof

ABSTRACT

The object of the present invention is to provide a material excellent in field electron emission which can withstand the high intensity of electric field, allows the enhanced emission of electrons resulting in a high density of current, and does not degrade during long use.  
     The solving means consists of providing a membrane body of sp 3 -bonded boron nitride excellent in field electron emission obtained by a method comprising the steps of introducing a reactive gas including a boron source and a nitrogen source into a reaction system; adjusting the temperature of a substrate in the reaction chamber to fall between room temperature to 1300° C.; radiating a UV beam onto the substrate with or without the concomitant existence of plasma; and forming via vapor-phase reaction a membrane on the substrate in which a surface texture allowing excellent field electron emission is formed in a self-organized manner.

TECHNICAL FIELD

The present invention relates to a membrane body of boron nitride, orsp³-bonded boron nitride whose structure is represented by generalformula BN, which has a surface texture excellent in field electronemission, method for producing such a membrane body and use thereof.

More specifically, the present invention relates to the aforementionednovel material which allows the formation of a film having anextraordinarily high field electron emission ability (the density ofresulting current being 1000 times or more as high as that of acorresponding conventional film) which is intended to be applied in thefield of lamp-type light source devices based on field electronemission, or field emission type displays.

BACKGROUND ART

Recently, in the technical fields related with the materials capable ofelectron emission ability, various materials capable of electronemission have been proposed, and those that can withstand high voltageor allow the generation of high density current have been sought. As oneof the materials that satisfy such demands, carbon nanotube attractsrecently the attention of people concerned. However, if carbon nanotubeis to be used as a material for producing a film suitable for electronemission, it will be necessary to devise a new technique whereby one canenhance its electron emission ability, that is, to raise the density ofresulting current.

Therefore, attempts have been made to enhance the electron emissionability of a nanotube film by growing nanotubes into a film with apattern specifically aimed at the enhanced emission of electrons, ormodifying a nanotube film using print transcription technique so that ithas a form suitable for the enhanced emission of electrons.

However, the nanotube film obtained as a result of such laborious,exquisite technique only allows the generation of electric current whosedensity is at most in the order of mA/cm². In addition, the film imposesa limitation for the intensity of applied voltage, and if appliedvoltage were higher than that limitation, the film would degrade soonand its superficial layers be torn off, i.e., the film could notwithstand use under high voltage for a sufficiently long period of time.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

In view of the current situation described above, the present inventiontries to meet the demand manifest in the field electron emissiontechnique, thereby accelerating progress in this promising field. Putspecifically, the object of the present invention is to provide a novelmaterial capable of achieving stable field electron emission, that is,capable of withstanding the high intensity of electric field, stablyemitting a large amount of electrons in terms of the density of currentfor a long period of time without undergoing degradation or damage.

Means for Solving the Problems

To achieve the above object, the present inventors noticed boron nitridewhich has been used as a heat-resistant, anti-corrosive material, andrecently attracts attention as a newly elaborated material. They studiedhard to design a material capable of electron emission based on thematerial. They found that among boron nitride compounds produced under acertain condition, there are some that, when grown into a film, producea film having a surface texture that exhibits excellent field electronemission, and tolerance against the high intensity of applied electricfield.

The finding made and confirmed by the inventors is as follows. Boronnitride is allowed to deposit on a substrate via vapor-phase reaction.During this operation, when a high energy UV beam is radiated close tothe substrate, boron nitride deposits on the substrate in the form of amembrane, and discrete dots of protrusions grow on the film in aself-organized manner with a certain interval from each other, eachprotrusion having a sharp end directing upward. The resulting film, whenexposed to an electric field, readily emits electrons. In addition, thefilm maintains the high density of current, extraordinarily high ascompared with the corresponding value obtained heretofore fromconventional films, that is, maintains the highly stable condition andperformance without undergoing degradation or damage or torn-off, i.e.,the film is found to have a very excellent electron emission ability.

The present invention is achieved based on this finding, and provides amembrane of boron nitride whose surface has a physical condition ortexture so unique as to be never observed in the correspondingconventional films, and which ensures highly excellent electron emissionperformance, and a method for producing such a membrane, and succeeds indeveloping new applications based on the use of this unique material,the feature of the invention being composed of technical constituents asdescribed in the following paragraphs (1) to (11).

The technical constituents of the present invention are based on therequirements described in the following paragraphs (1) to (11).

(1) A membrane body of sp³-bonded boron nitride excellent in fieldelectron emission obtained by vapor-phase deposition in which a surfacetexture allowing excellent in field electron emission is formed in aself-organized manner.

(2) A membrane body of sp³-bonded boron nitride as described inparagraph (1) excellent in field electron emission wherein the surfacetexture allowing excellent field electron emission comprises discretedots of protrusions each having a sharp tip end.

(3) A membrane body of sp³-bonded boron nitride as described inparagraph (1) or (2) excellent in field electron emission wherein thediscrete dots of protrusions are separated from each other at aninterval or distributed at a density suitable for field electronemission.

(4) A membrane body of sp³-bonded boron nitride as described in any oneof paragraphs (1) to (3) excellent in field electron emission whichcomprises polytype boron nitride such as 5H type or 6H type boronnitride.

(5) A membrane body of sp³-bonded boron nitride as described in any oneof paragraphs (1) to (4) excellent in field electron emission which isformed on a substrate as a result of vapor-phase reaction excited by aUV beam.

(6) A method for producing a membrane body of sp³-bonded boron nitrideexcellent in field electron emission, characterized in comprising thesteps of introducing a reactive gas including a boron source and anitrogen source whose pressure is adjusted to 0.001 to 760 Torr into areaction system; adjusting the temperature of a substrate in thereaction chamber to fall between room temperature and 1300° C.;radiating a UV beam onto the substrate with or without the concomitantexistence of plasma; and forming via vapor-phase reaction a membrane onthe substrate in which a surface texture allowing excellent fieldelectron emission is formed in a self-organized manner.

(7) A method as described in the paragraph (6) for producing a membranebody of sp³-bonded boron nitride excellent in field electron emissionwherein the reaction gas is obtained via the dilution by a diluting gassuch as a rare gas or hydrogen gas or their mixture, the dilutionoccurring by mixing the reaction gas with the diluting gas at a volume(%) ratio of 0.0001-100 to 100.

(8) A method as described in the paragraph (6) or (7) for producing amembrane body of sp³-bonded boron nitride excellent in field electronemission wherein the reactive gas comprises diborane as a boron sourceand ammonia as a nitrogen source.

(9) A method as described in the paragraph (6) for producing a membranebody of sp³-bonded boron nitride excellent in field electron emission,characterized in wherein the UV beam occurs as pulsed laser.

(10) A method as described in any one of paragraphs (6) to (9) forproducing a membrane body of sp³-bonded boron nitride excellent in fieldelectron emission, characterized in wherein the membrane body ofsp³-bonded boron nitride excellent in field electron emission comprisespolytype boron nitride such as 5H type or 6H type boron nitride.

(11) A membrane body of sp³-bonded boron nitride as described in any oneof paragraph (1) to (4) excellent in field electron emission which isused as a material for electron emission.

EFFECT OF THE INVENTION

The membrane body of sp³-bonded boron nitride excellent in fieldelectron emission which has a surface texture with discrete dots ofprotrusions each having a sharp tip end provided by this inventiondistributed thereon ensures the following advantages:

(1) It is low in the threshold of field electron emission;

(2) It allows the high density of current; and

(3) It has a long life in field electron emission.

The inventive material is superior as a material for field electronemission to standard conventional materials. The inventive material isparticularly noteworthy in the advantages (2) and (3) (it ensures thedensity of current 1000 times or more higher than the conventionalmaterial, and is structurally robust and resistant as is characteristicwith BN). Thus, it will bring about never-been-observed-beforebreakthrough if it is applied as a material in the construction oflamp-type light source devices or field emission type displays, whichwill be very meaningful for those concerned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for showing the outline of a reaction system of theinvention and schematic representation of its operation.

FIG. 2 is a scanning electron-microscopy (SEM) photograph of a filmprepared in Example 1 in which the formation of a self-organized texturesuitable for field electron emission is confirmed on the surface.

FIG. 3 is a more enlarged SEM photograph of a film prepared in Example1.

FIG. 4 is a graph for showing the field electron emission property of afilm prepared in Example 1.

FIG. 5 is a graph for showing the stableness over time of the fieldelectron emission of a film prepared in Example 1 plotted as a functionof current density.

FIG. 6 is a graph for showing the field electron emission property of afilm prepared in Comparative Example where no UV beam is used.

FIG. 7 is an SEM photograph of a film prepared in Comparative Examplewhere no UV beam is used which was broken during experiment performed tostudy its field electron emission.

FIG. 8 is an SEM photograph of a film prepared in Example 2 in which theformation of a self-organized texture suitable for field electronemission is confirmed on the surface.

FIG. 9 is a graph for showing the field electron emission property of afilm prepared in Example 2.

REFERENCE NUMERALS

1: Reaction chamber (reaction furnace)

2: Gas inlet

3: Gas outlet

4: Substrate upon which boron nitride is to be deposited

5: Optical window

6: Exima UV laser unit

7: Plasma torch

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiments of the present invention will be described indetail below with reference to the attached drawings and Examples. FIG.1 shows a reaction system used according to the present invention forobtaining a membrane body of sp³-bonded boron nitride excellent in fieldelectron emission. As shown in the FIG. 1, the reaction system 1comprises a gas inlet 2 for introducing a reaction gas and diluting gas,and a gas outlet 3 for evacuating the reaction gas out of the reactionchamber, and is connected to a vacuum pump in such a manner as to allowthe pressure within the chamber to be maintained below the atmosphericpressure. Within the chamber, along the flow passage of gas there isprovided a substrate 4 on which boron nitride will be deposited. On thewall of chamber facing the boron nitride deposition surface ofsubstrate, there is provided an optical window 5 to which an exima UVlaser unit 6 is placed such that UV light therefrom is radiated throughthe window onto the boron nitride deposition surface of substrate.

The reaction gas introduced into the reaction system is excited as aresult of the exposure to UV light directed towards the active surfaceof substrate so that a nitrogen source and boron source therein undergovapor-phase reaction to cause sp³-bonded boron nitride, and a generalformula;BN is indicated, having a 5H or 6H polytype of structure to beformed and deposited on the active surface of substrate, the depositiongrowing into a membrane thereon.

As a result of experiment, it is revealed that the formation of membraneis possible even when the pressure within the reaction system varies ina wide range of 0.001 to 760 Torr, or when the temperature within thereaction system varies in a wide range of room temperature to 1300° C.However, to obtain the desired reaction product having a high purity, itis preferable to maintain the pressure at a low level and thetemperature at a high level.

A case where, when a UV light is radiated to the active surface ofsubstrate and its surrounding space, plasma is also evoked so as to beexposed to the UV light forms another embodiment of the invention.Indeed, in FIG. 1, the plasma torch 7 is introduced to realize such anembodiment where the inlet for introducing reaction gas and plasma torchare integrally combined with respect to the substrate such that reactiongas and plasma are directed towards the active surface of substrate.

The boron nitride membrane of the invention is produced using a reactionsystem as described above, and the involved procedures will be describedbelow with reference to attached figures and concrete Examples. However,these Examples are disclosed only as an aid for facilitating the readyappreciation of the present invention. It should be understood that thescope of the invention is not limited in any way by those Examples.

In summary, the present invention aims to provide a method for producinga membrane body of sp³-bonded boron nitride excellent in field electronemission obtained by vapor-phase deposition in which a surface textureallowing excellent field electron emission is produced in aself-organized manner, and new applications where the membrane body isutilized as an electron emission material. The condition under which theobject of the invention is achieved is not determined uniquely but canvary as appropriate according to given situations, and the scope of theinvention includes such variations.

EXAMPLE 1

Into a mixed dilution gas comprising argon gas flowing at 2 SLM andhydrogen gas flowing at 50 sccm were introduced diborane gas flowing at10 sccm and ammonia gas flowing at 20 sccm, and at the same time, anexima UV laser was radiated to a silicon substrate kept at 800° C. byheating in a chamber of which the pressure of atmosphere was kept at 30Torr by a pump engaged in the evacuation of the atmosphere (see FIG. 1).When 60 minutes were allowed to pass for synthesis, a target film wasobtained. This film product was examined by the X-ray diffraction methodwhich revealed that the material consisted of hexagonal crystal, and hadan sp³-bonded, 5H polytype structure. Its lattice constants were a=2.52Å, and c=10.5 Å.

The results obtained by observing the material by scanningelectron-microscopy (SEM) are represented in FIGS. 2 and 3. As seen fromthe observation results shown in the figures, it was demonstrated thatthe film obtained by the inventive method has a characteristic surfacetexture comprising discrete dots of conical protrusions each having asharp tip end (several to several tens microns in length) formed in aself-organized manner, each protrusion being likely to serve as a spotwhere electric field concentrates. Incidentally, FIG. 3 is an enlargedSEM photograph of the same photograph shown in FIG. 2.

To evaluate the field electron emission of this film, a metalcylindrical electrode having a diameter of 1 mm was placed 30 micrometerapart from the surface of the film in a vacuum chamber, and a voltage isapplied between the film and the electrode, and electron emission fromthe film was measured.

As a result, it was demonstrated that the film exhibits a characteristicas shown in FIG. 4. According to the graph shown in the figure, for theboron nitride film of the invention, the density of current starts toincrease at the field intensity of 15-20 (V/μm), and reaches, at thefield intensity of 20 (V/μm), a saturation level (equal to 1.3 A/cm²)which corresponds to the upper limit permitted for a high voltage powersource used for measurement.

Then, when the field intensity was kept at a level where the currentintensity value reached a saturation level, the change of currentintensity value over time was followed. The result is shown in FIG. 5.Although more or less undulations were observed at around 900 sec (15minutes), the current intensity values were fairly stabilized around theaverage, that is, the current intensity value did not show any declineindicative of the degradation of material, which demonstrates that thematerial is stable.

COMPARATIVE EXAMPLE 1

As a comparison, a film was prepared in parallel with the one of Example1 under the same condition, except that it did not receive theirradiation of UV beam, and for a portion of the film where no UV beamwas irradiated, its field electron emission was evaluated. The result isshown in FIG. 6. It was demonstrated that, for the comparative film, thethreshold intensity of field responsible for the start of electronemission is 42 (V/μm) which is far higher than the corresponding value15 (V/μm)) obtained for the experimental film which received theirradiation of UV beam.

As seen from the SEM photograph shown in FIG. 7, the film materialsuffered damages and torn-offs after the comparative test. By contrast,another portion of the comparative film which had a characteristicsurface texture comprising protrusions grown as a result of the exposureto UV beam did not show such damages as described above after the samefield electron emission evaluation test.

EXAMPLE 2

The same reaction system with that used in Example 1 was used. Into amixed dilution gas comprising argon gas flowing at 2 SLM and hydrogengas flowing at 50 sccm were introduced diborane gas flowing at 10 sccmand ammonia gas flowing at 20 sccm in the system, and at the same time,the pressure of atmosphere in the chamber was kept at 30 Torr by a pumpengaged in the evacuation of the atmosphere, an RF plasma with an outputof 800 w and frequency of 13.56 MHz was evoked in the atmosphere, and anexima UV laser was radiated for 60 minutes to a silicon substrate keptat 900° C. by heating, to allow boron nitride to deposit on thesubstrate (see FIG. 1).

As a consequence, a film product was obtained. The structure of theproduct was determined by the same method as in Example 1, and as aresult it was demonstrated that the product consisted of hexagonalcrystal, and had an sp³-bonded, 5H polytype structure. Its latticeconstants were a=2.5 Å, and c=10.4 Å.

The result obtained by observing the product by SEM is as shown in FIG.8. As seen from the figure, it was demonstrated that the film thusproduced bears discrete dots of conical protrusions each having a sharptip end (several to several tens microns in length) which is likely toserve as a spot where electric field concentrates, that is, the surfaceof the substrate is covered with such protrusions. Namely, it wasdemonstrated by this example that the characteristic surface texture isformed in a self-organized manner.

To evaluate the field electron emission of this film, a metalcylindrical electrode having a diameter of 1 mm was placed 40 micrometerapart from the surface of the film in a vacuum chamber, and a voltage isapplied between the film and the electrode, and electron emission fromthe film was measured. As a result of the study, the data shown in FIG.9 were obtained. Specifically, for the film under study, the density ofcurrent starts to increase at the field intensity of 18-22 (V/μm), andreaches, at the field intensity of 22 (V/μm), a saturation level (equalto 1.3 A/cm²) which corresponds to the upper limit permitted for a highvoltage power source used for measurement. The result showed that it isalso possible to obtain a stable material as with the method of Example1.

As seen from above, it was demonstrated that it is possible to obtain aboron nitride membrane via vapor-phase deposition performed under acondition characteristic with the present invention, in which a surfacetexture allowing excellent field electron emission is formed in aself-organized manner. It was also demonstrated the condition includes,as a necessary element, the irradiation of UV beam. This is apparent byreferring to the difference between the Examples and the ComparativeExample. However, the reason why the difference in condition between thetwo modes of processings lead to the creation of products so differentin their characteristics as seen above remains unclear as far as basedon the present state of knowledge.

However, if it were possible to assume the reaction mechanism asdescribed below, the difference in question could be explained.

According to the suggestion given by Ilya Prigogine (Nobel prize winner)and others, formation of a surface texture in a self-organized mannercan be understood as a phenomenon similar to the formation of a “Turingstructure” which appears under a condition where the superficialdispersion of a precursor substance and the superficial chemicalreaction compete with each other. In case of the invention, irradiationof UV light photochemically accelerates both processes, which will havean effect on the orderly arrangement of initial nuclei.

Crystal growth reaction on the surface is accelerated by the irradiationof UV light, which means that the reaction velocity increases inproportion to the intensity of light. If it were assumed that each ofthe initial nuclei has a semicircular shape, the summit and itsenvirons, being exposed to the comparatively high intensity of light,are accelerated in their growth, while the periphery of semicircle,being exposed to the comparatively weak light, is retarded in itsgrowth, which will explain why the semicircular nucleus grow intoprotrusions with a sharp tip end which determines the surface texture ofthe film of the invention.

In anyway, it can not be denied that for the formation of an inventivefilm, irradiation of UV light plays a key role and acts as a crucialfactor.

As described above, the present invention provides a membrane body ofsp³-bonded boron nitride in which a surface texture allowing excellentfield electron emission is formed in a self-organized manner, thesurface texture comprising discrete dots of protrusions each having asharp tip end. Thus, it becomes possible to obtain an ideal, novelmaterial which has a low threshold in field electron emission, allowsthe passage of high density of current, and ensures a long life in fieldelectron emission without requiring any special processing means andprocedures, which is very important.

INDUSTRIAL APPLICABILITY

A membrane body of sp³-bonded boron nitride excellent in field electronemission provided by the present invention has a spectacularsignificance as described above, and the range of uses and applicationsit commands is very wide and varied as is obvious from the list citedbelow. Thus, it is expected that the material will be exploited andincorporated in a wide variety of technical fields, thereby contributingto the progress of industry.

Since the membrane body provided by the present invention emitselectrons which are 1000 times or more in terms of the density ofcurrent than those from a conventional field electron emission material,it will allow the construction of an illumination system exhibiting aultra-high brightness at a high efficiency, or of a ultra-fineresolution display based on minute pixels each allowing the flow ofsufficiently high current in spite of its being small in size (thedisplay will be profitably incorporated in a mobile phone, ultra tinycomputer, etc.), and formation of a characteristic electron emissionpattern exploiting the phenomenon of a film in which one part irradiatedby UV light has a higher electron emission than that of another part notirradiated by UV light. In addition, the membrane body will open the wayfor the construction of a nano-size electron beam source having a ultrahigh brightness, ultra-small electron beam source, etc. As a result, itsfuture application will include not only illuminations and displays, butalso various electric appliances and devices used in everyday life inthe modern world, and revolutionize them. In short, the material of theinvention commands such a rich possibility that it will permeate throughthe every corner of human life, to effect a global impact in technologyand economics.

Furthermore, The present invention is based on the discovery of a uniquephenomenon related with the formation of a film under the exposure oflight in which a characteristic surface texture develops in aself-organized manner. Even if the film is used neat without beingprocessed, the film with the surface texture allows the markedlyenhanced emission of electrons. In addition, by the physical propertiescharacteristic with the material of the film, the film is essentiallyfree from damages due to emission of electrons, even when it maintainsthe flow of high density of current. If it is assumed that the filmincorporated in one of the applications described above, to work thereindefinitely for a permanent period, its utility as compared with theconventional equivalent material will be immense, because it will notonly ensure a far longer life, but also dispense with many steps andprocedures involved in the processing and patterning for field electronemission required for those conventional materials. The advent of theinventive material will ensure a leap in the development of relatedtechnology.

In summing up, the present invention provides a novel material or a filmwhose surface takes a characteristic texture in a self-organized manner,the film having an excellent field electron emission as a result of thecumulative effect due one part to the surface texture and the other partto the excellent physical property of the film material itself, theexcellent field emission being represented by the density of current inthe order of A/cm² that is 1000 times or more higher than that of acorresponding film, and the excellent physical property of the filmmaterial represented by its excellent durability. The present inventionalso provides the method for producing the material and the use thereof.These features of the invention will prove to be revolutionary bybringing a breakthrough on the current technical standards.

1. A membrane body of sp³-bonded boron nitride excellent in fieldelectron emission obtained by vapor-phase deposition in which a surfacetexture allowing excellent in field electron emission is formed in aself-organized manner.
 2. A membrane body of sp³-bonded boron nitride asdescribed in claim 1 excellent in field electron emission wherein thesurface texture allowing excellent in field electron emission comprisesdiscrete dots of protrusions each having a sharp tip end.
 3. A membranebody of sp³-bonded boron nitride as described in claim 1 excellent infield electron emission wherein the discrete dots of protrusions areseparated from each other at an interval or distributed at a densitysuitable for field electron emission.
 4. A membrane body of sp³-bondedboron nitride as described in claim 1 excellent in field electronemission, characterized in which comprises polytype boron nitride suchas 5H type or 6H type boron nitride.
 5. A membrane body of sp³-bondedboron nitride as described in claim 1 excellent in field electronemission which is formed on a substrate as a result of vapor-phasereaction excited by a UV beam.
 6. A method for producing a membrane bodyof sp³-bonded boron nitride excellent in field electron emission,characterized in comprising the steps of introducing a reactive gasincluding a boron source and a nitrogen source whose pressure isadjusted to 0.001 to 760 Torr into a reaction system; adjusting thetemperature of a substrate in the reaction chamber to fall between roomtemperature and 1300° C.; radiating a UV beam onto the substrate with orwithout the concomitant existence of plasma; and forming via vapor-phasereaction a membrane on the substrate in which a surface texture allowingexcellent field electron emission is formed in a self-organized manner.7. A method as described in claim 6 for producing a membrane body ofsp³-bonded boron nitride excellent in field electron emission whereinthe reaction gas is obtained via the dilution by a diluting gas such asa rare gas or hydrogen gas or their mixture, the dilution occurring bymixing the reaction gas with the diluting gas at a volume ratio of0.0001-100 to
 100. 8. A method as described in claim 6 for producing amembrane body of sp³-bonded boron nitride excellent in field electronemission wherein the reactive gas comprises diborane as a boron sourceand ammonia as a nitrogen source.
 9. A method as described in claim 6for producing a membrane body of sp³-bonded boron nitride excellent infield electron emission, characterized in which the UV beam occurs aspulsed laser.
 10. A method as described in claim 6 for producing amembrane body of sp³-bonded boron nitride excellent in field electronemission, characterized wherein the membrane body of sp³-bonded boronnitride excellent in field electron emission comprises polytype boronnitride such as 5H type or 6H type boron nitride.
 11. A method asdescribed in claim 1 for producing a membrane body of sp³-bonded boronnitride excellent in field electron emission, characterized in which isused as a material for electron emission.